In recent years, various proposals have been made to develop steer-by-wire systems that use an electric or hydraulic actuator for steering either front wheels or rear wheels to enable the corresponding wheels to be steered without using a mechanical coupling between a steering wheel and the road wheels that are steered. See Japanese patent laid open publication (kokai) No. 2004-50978 and Japanese patent laid open publication (kokai) No. 05-131945. When a steer-by-wire system is used, the lateral acceleration (yaw rate) property of the vehicle at the time of cornering and changing lanes can be determined as desired by using a lateral acceleration model following control, yaw rate model following control or the like.
According to simulations (theoretical computations) conducted by the inventors, when a lateral acceleration model following control (or yaw rate model following control) is used, it was found that the lateral acceleration response and the settling property thereof in a high frequency range of the steering input can be improved, but a certain problem arises.
FIG. 9 shows graphs representing frequency responses (lateral acceleration and yaw rate response), in terms of both gain and phase, to a steering input when a yaw rate following control is used. As can be appreciated from these graphs, the yaw rate following control improves both the yaw rate response and settling property thereof (indicated by the solid line) as compared to the case where the yaw rate following control is not used (indicated by the dotted line). However, some advance in phase occurs to the lateral acceleration in a high steering frequency range, for instance, as indicated by the dotted circle in FIG. 9, and this is perceived by a vehicle occupant as a delayed disturbance following the settling of the motion of the vehicle which may cause a discomfort to the vehicle occupant. In other words, depending on the extent of the desired improvement in the yaw rate response, the timing of a rise in lateral acceleration may become unstable, and this in turn may cause an unstable rolling motion of the vehicle.